Controlling wind and solar farms for reliable grids

Project state

Started

Grid stability given high converter penetration

The main objective of this research project is to develop a control method for electricity from renewable energy sources and loads so that grid stability is retained even at very high shares of renewable electricity. This requires novel solutions as more and more power converter-based generators and loads are being connected to the European interconnected network. Legislation currently in force requires a minimum share of conventional power plants. In the future, they will be required less often to meet the demand for electricity.

The so-called conventional minimum production is a burden on the climate and leads to high costs. On the other hand, major disturbances resulting in grid separation, such as occurred on 4 November 2006 or 28 July 2003, are no longer reliably controllable with today’s control methods and the regionally high proportion of renewable energy. The control methods created during the project are designed to ensure that grid operation becomes technically independent of conventional power plants, thus realising the planned expansion targets for renewable energies. The project’s focus is on transient stability in the short-term range – i.e. up to about thirty seconds after the beginning of a fault event.

The picture shows the schematic separation of a sub-grid from the interconnected grid. This places special demands on converter control. Source: Fraunhofer IWES

The project is carried out jointly by the partners Fraunhofer IWES Kassel, SMA Solar Technology AG, the professorship for Control and System Theory at the University of Kassel, and the chair of Power Electronics at the University of Rostock. Fraunhofer IWES, Institute for Energy System Technology in Kassel, focusses on examining the approach with the so-called virtual synchronous machine for wind turbines, and coordinates the planned experiments. The team led by Prof. Dr.-Ing. Olaf Stursberg at the University of Kassel add their expertise on the subject of model predictive control for wind farms. The inverter manufacturers SMA are developing control methods for solar inverters and solar farms, and Prof. Dr.-Ing. Hans-Günter Eckel’s department in Rostock is researching methods for loads connected via power electronics. The partners launched their joint project in Kassel in June 2015, where they held a kick-off meeting.

Development of control methods and demonstration within the test grid

The planned tests of the research project Transstabil-EE will take place in the medium-voltage test environment. Source: Fraunhofer IWES

Before the engineers develop the control method, they first need reference scenarios that allow them to later check the controllers. One such scenario could be, for example, a large-scale disturbance, such as occurred during the grid separation event in 2006. Furthermore, the project participants create appropriate models to design the controller and the subsequent validation. A substantial portion of the project effort is devoted to the development of the control methods, which are initially drawn up independently of the technology used for the power converters. Next, the engineers add specific extensions for the respective plant technology. This way, wind turbines, wind and solar farms, and controllable loads are more easily integrated in the control scheme. Experimental facilities are being built at SMA and IWES for the investigations, in which the methods can be experimentally validated. The medium-voltage test grid at IWES is being extended so that a scaled wind farm with the power electronics of two wind turbines is created. The aerodynamics and structural dynamics of the wind turbines are simulated as virtual systems. In addition, methods for solar farms and controllable loads are being integrated.

On one hand, the expectable results of the project are control methods that allow for grid operation without conventional power plants. On the other, there will be available studies on the future operation of power grids with a high proportion of renewable energy sources - especially with respect to making do without synchronous machines.